In the manufacture of semiconductor wafers, microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle. Reticles and masks typically consist of an opaque thin film of a metal, such as chromium, deposited in a pattern on a transparent substrate of quartz or glass. During the exposure process a form of radiant energy, such as ultraviolet light, is directed through the template to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist pattern on the wafer. The resist pattern can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
Manufacturers in the field of integrated circuits (ICs) have been trying to reduce the geometric size of the devices on integrated circuits. The benefits achieved by reducing device dimensions include higher performance of circuit elements and smaller packaging sizes. Improving lithographic techniques provide improved resolution and results in a potential reduction of device dimensions. However, at small geometries, diffraction effects such as proximity effects, poor subject contrast, and poor resolution result, producing wafers with incomplete or erroneous circuit patterns.
A lithographic technique useful at small geometries is known as phase shifting lithography. In phase shifting lithography, the interference between waves of exposure energy is used to overcome diffraction effects and to improve the resolution and depth of optical images projected onto a target. Phase shifting lithography involves controlling the phase of exposure light at the target such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the target (i.e., wafer) and allows resolutions as fine as 0.10 microns to occur.
Phase shifting areas in a mask are typically formed by varying the thickness of one transparent portion of the mask with respect to another transparent portion. Thickness of transparent portions is usually reduced by etching. The etch is carefully controlled to create trenches having a specified depth so light passing through the region with a trench passes through a quartz substrate thinned compared to light passing through a neighboring region that has the fill thickness of quartz, the difference in thickness providing a 180° phase difference in the light transmitted by each. Because of the interference of the out of phase light there is a significant intensification of contrast between adjacent regions when light is shined through the mask, and this intensification of contrast provides the ability to resolve significantly smaller structures than can be achieved without phase shifting. Typically, the etch depth is controlled to provide a relative shift between the two neighboring regions of half of the wavelength of the light used with the mask to expose semiconductor wafers, thereby providing the 180° phase difference. Masks having other phase differences, e.g., 90° or 270°, have also been produced.
Etching and associated processes often leave behind defects in masks. Opaque defects, which may occur as spots, pattern extensions, bridges between adjacent patterns, or the like, are the result of opaque material such as chromium or molybdenum silicide being present in a non-pattern area. Clear defects, which generally occur as bumps, pinholes, missing parts, or breaks in the pattern, can result from missing or inadequate layers of opaque material in a pattern area on the template.
Focused ion beams (FIBs) have been used for repair of optical masks and reticles since the mid-1980s. The ability of the FIB to accurately remove unwanted portions of the metal film and to deposit material to “edit” the pattern makes it potentially an almost ideal repair tool. A FIB exposes a template to a beam of positively charged ions, typically gallium ions, via an optical system. When a template is exposed to the ion beam, secondary ions and electrons are produced, and may be detected by the FIB machine and monitored to determine the progress of repair work. If a chromium pattern is exposed, secondary chromium ions are generated, and if a silicon or glass pattern is exposed, secondary silicon ions are generated.
Analogous to opaque defects are regions where quartz should have been etched but was not. In these regions of the mask there remain unwanted areas of full or partial thickness quartz. Defects are likely in the quartz due to a mechanism such as foreign material falling on the mask before or during the quartz etch step that follows chrome repair. When FIB machines are used to repair such defects, undesirable byproducts are often generated. The high energy focused ion beam used to remove the quartz bump defect can cause damage to areas of the phase shifting mask surrounding the defect. This is usually due to scattering of the ion beam after it imparts the quartz bump defect, or due to inaccurate alignment of the ion beam.
Thus, a solution is needed that provides for accurately and reliably repairing defects in the quartz of phase shift masks, while minimizing damage to the substrate.